A typical Li-ion cell contains a negative electrode, the anode, a positive electrode, the cathode, and a separator region between the negative and positive electrodes. One or both of the electrodes contain active materials that react with lithium reversibly. In some cases the negative electrode may include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator and positive electrode contain an electrolyte that includes a lithium salt.
Charging a Li-ion cell generally entails a generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode with the electrons transferred via an external circuit. In the ideal charging of the cell, these electrons are generated at the positive electrode because there is extraction via oxidation of lithium ions from the active material of the positive electrode, and the electrons are consumed at the negative electrode because there is reduction of lithium ions into the active material of the negative electrode. During discharging, the opposite reactions occur.
Li-ion cells with a Li-metal anode may have a higher specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. This high specific energy and energy density makes incorporation of rechargeable Li-ion cells with a Li-metal anode in energy storage systems an attractive option for a wide range of applications including portable electronics and electric and hybrid-electric vehicles.
At the positive electrode of a conventional lithium-ion cell, a lithium-intercalating oxide is typically used. Lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, Li1.1Ni0.3CO0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g, which is quite low compared to the specific capacity of lithium metal (3863 mAh/g).
Moreover, the low realized capacities of conventional Li-ion cells reduces the effectiveness of incorporating Li-ion cells into vehicular systems. Specifically, a goal for electric vehicles is to attain a range approaching that of present-day vehicles (>300 miles). Obviously, the size of a battery could be increased to provide increased capacity. The practical size of a battery on a vehicle is limited, however, by the associated weight of the battery. Consequently, the Department of Energy (DOE) in the USABC Goals for Advanced Batteries for EVs has set a long-term goal for the maximum weight of an electric vehicle battery pack to be 200 kg (this includes the packaging). Achieving the requisite capacity given the DOE goal requires a specific energy in excess of 600 Wh/kg.
Various materials are known to provide a promise of higher theoretical capacity for Li-based cells. For example, a high theoretical specific capacity of 1168 mAh/g (based on the mass of the lithiated material) is shared by Li2S and Li2O2, which can be used as cathode materials. Other high-capacity materials include BiF3 (303 mAh/g, lithiated) and FeF3 (712 mAh/g, lithiated) as reported by Amatucci, G. G. and N. Pereira, “Fluoride based electrode materials for advanced energy storage devices,” Journal of Fluorine Chemistry, 2007. 128(4): p. 243-262. Unfortunately, all of these materials react with lithium at a lower voltage compared to conventional oxide positive electrodes. Nonetheless, the theoretical specific energies are still very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).
One Li-based cell that has the potential of providing a driving range above 300 miles incorporates a lithium metal negative electrode and a positive electrode reacting with oxygen obtained from the environment or an onboard O2 storage tank. The weight of this type of system may be reduced if the O2 is not carried onboard the vehicle. Practical embodiments of this lithium-air battery may achieve a practical specific energy of 600 Wh/kg because the theoretical specific energy is 11,430 Wh/kg for Li metal, and 3,460 Wh/kg for Li2O2.
During discharge of the lithium-air cell, Li ions are stripped from the Li-metal negative electrode, while at the positive electrode, lithium ions (Li+ ions) in the electrolyte react with oxygen and electrons to form a solid discharge product that ideally is lithium peroxide (Li2O2) or lithium oxide (Li2O), which may coat the conductive matrix of the positive electrode and/or fill the pores of the electrode. In an electrolyte that uses a carbonate solvent the discharge products may include Li2CO3, Li alkoxides, and Li alkyl carbonates. In non-carbonate solvents such as CH3CN and dimethyl ether the discharge products are less likely to react with the solvent. The pure crystalline forms of Li2O2 and Li2O are electrically insulating, so that electronic conduction through these materials will need to involve vacancies, grains, or dopants, or short conduction pathways obtained through appropriate electrode architectures.
Abraham and Jiang published one of the earliest papers on the “lithium-air” system. See Abraham, K. M. and Z. Jiang, “A polymer electrolyte-based rechargeable lithium/oxygen battery”; Journal of the Electrochemical Society, 1996. 143(1): p. 1-5. Abraham and Jiang used an organic electrolyte and a positive electrode with an electrically conductive carbon matrix containing a catalyst to aid with the reduction and oxidation reactions. Previous lithium-air systems using an aqueous electrolyte have also been considered, but without protection of the Li metal anode, rapid hydrogen evolution occurs. See Zheng, J., et al., “Theoretical Energy Density of Li-Air Batteries”; Journal of the Electrochemical Society, 2008. 155: p. A432.
An electrochemical cell 10 is depicted in FIG. 1. The cell 10 includes a negative electrode 12, a positive electrode 14, a porous separator 16, and a current collector 18. The negative electrode 12 is typically metallic lithium. The positive electrode 14 includes carbon particles such as particles 20 possibly coated in a catalyst material (such as Au or Pt) and suspended in a porous, electrically conductive matrix 22. An electrolyte solution 24 containing a salt such at LiPF6 dissolved in an organic solvent such as dimethyl ether or CH3CN permeates both the porous separator 16 and the positive electrode 14. The LiPF6 provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the cell 10 to allow a high power.
The positive electrode 12 is enclosed by a barrier 26. The barrier 26 in FIG. 1 is formed from an aluminum mesh configured to allow oxygen from an external source 28 to enter the positive electrode 14. The wetting properties of the positive electrode 14 prevent the electrolyte 24 from leaking out of the positive electrode 14. Oxygen from the external source 28 enters the positive electrode 14 through the barrier 26 while the cell 10 discharges, and oxygen exits the positive electrode 14 through the barrier 26 as the cell 10 is charged. In operation, as the cell 10 discharges, oxygen and lithium ions combine to form a discharge product such as Li2O2 or Li2O.
A number of investigations into the problems associated with Li-air batteries have been conducted as reported, for example, by Beattie, S., D. Manolescu, and S. Blair, “High-Capacity Lithium-Air Cathodes,” Journal of the Electrochemical Society, 2009. 156: p. A44, Kumar, B., et al., “A Solid-State, Rechargeable, Long Cycle Life Lithium-Air Battery,” Journal of the Electrochemical Society, 2010. 157: p. A50, Read, J., “Characterization of the lithium/oxygen organic electrolyte battery,” Journal of the Electrochemical Society, 2002. 149: p. A1190, Read, J., et al., “Oxygen transport properties of organic electrolytes and performance of lithium/oxygen battery,” Journal of the Electrochemical Society, 2003. 150: p. A1351, Yang, X and Y. Xia, “The effect of oxygen pressures on the electrochemical profile of lithium/oxygen battery,” Journal of Solid State Electrochemistry: p. 1-6, and Ogasawara, T., et al., “Rechargeable Li2O2 Electrode for Lithium Batteries,” Journal of the American Chemical Society, 2006. 128(4): p. 1390-1393. Nonetheless, several challenges remain to be addressed for lithium-air batteries. These challenges include limiting dendrite formation at the lithium metal surface, protecting the lithium metal (and possibly other materials) from moisture and other potentially harmful components of air, designing a system that achieves acceptable specific energy and specific power levels, reducing the hysteresis between the charge and discharge voltages (which limits the round-trip energy efficiency), and improving the number of cycles over which the system can be cycled reversibly.
The limit of round trip efficiency occurs due to an apparent voltage hysteresis as depicted in FIG. 2. In FIG. 2, the discharge voltage 40 (approximately 2.5 to 3 V vs. Li/Li+) is much lower than the charge voltage 42 (approximately 4 to 4.5 V vs. Li/Li+). The equilibrium voltage 44 (or open-circuit potential) of the lithium/air system is approximately 3 V. Hence, the voltage hysteresis is not only large, but also very asymmetric.
The large over-potential during charge may be due to a number of causes. For example, reaction between the Li2O2 and the conducting matrix 22 may form an insulating film between the two materials. Additionally, there may be poor contact between the solid discharge products Li2O2 or Li2O and the electronically conducting matrix 22 of the positive electrode 14. Poor contact may result from oxidation of the discharge product directly adjacent to the conducting matrix 22 during charge, leaving a gap between the solid discharge product and the matrix 22.
Another mechanism resulting in poor contact between the solid discharge product and the matrix 22 is complete disconnection of the solid discharge product from the conducting matrix 22. Complete disconnection of the solid discharge product from the conducting matrix 22 may result from fracturing, flaking, or movement of solid discharge product particles due to mechanical stresses that are generated during charge/discharge of the cell. Complete disconnection may contribute to the capacity decay observed for most lithium/air cells. By way of example, FIG. 3 depicts the discharge capacity of a typical Li/air cell over a period of charge/discharge cycles.
There is additional evidence that the electronically insulating discharge products in lithium/air cells result in passivation of the electronically conducting substrate of the positive electrode, thereby severely limiting the discharge capacity far below what is required to approach the desired and theoretically achievable specific energy of Li/air cells as reported by Albertus et al., “Identifying Capacity Limitations in the Li/Oxygen Battery Using Experiments and Modeling,” Journal of the Electrochemical Society, 158, A343 (2011).
What is needed therefore is an energy storage system that can overcome limitations resulting from generation of an insulating LiO2 or Li2O2 film during discharging of a cell.